1. Field of the Invention
The present invention relates to an image formation method for forming an electrostatic latent image on an electrostatically charged photosensitive belt with a laser beam and an image formation apparatus of the same, more particularly to an image forming method which is capable of deciding a position for starting an image writing in a main scanning direction with a laser beam and an image formation apparatus of the same.
2. Description of the Prior Art
A laser printer, a copying machine and like using a photoconductive belt electrostatically charge a photosensitive body by a charger, and then form an electrostatic latent image on the photosensitive belt by scanning a laser beam including image information thereon, thus forming an electrostatic image. Regarding the electrostatic image, the electrostatic image is developed by a developing device and a developed image is formed by attaching toner onto the photosensitive belt. The developed image is transferred onto a recording paper, so as to record the image.
When a belt-shaped photosensitive body is used for the image formation using the laser beam, that is, for the formation of the electrostatic image, an oblique movement of the photosensitive belt or a slip of the photosensitive belt in a main scanning direction laser beam scanning direction) may occur. Therefore, the edge of the photosensitive belt slowly shifts to the main scanning direction. This is because when the photosensitive belt is driven by a roller separated from the belt, a relative positional offset in the main scanning direction by a slippage is apt to occur. Accordingly, there has been a problem that the position for staring the image writing by the laser beam is shifted with the movement of the photosensitive belt.
As a countermeasure to cope with such problem, as is recited in Japanese Patent Laid-open No. 4-274467 which was issued on Sep. 30, 1992, and Japanese Patent Laid-open No. 4-275574 which was issued on Oct. 1, 1992, presented is a method in which an edge portion (side edge) of a photosensitive belt in a main scanning direction is detected by a laser beam scanned during a latent image formation, and a position for stating writing in the main scanning direction is decided based on a detection signal detected by the laser beam.
Furthermore, a technology in which a sensor is disposed at an edge portion of a photosensitive belt in a main scanning direction (hereinafter, referred to a belt edge), and the belt edge is detected by the sensor as a reference position, thus controlling a timing of the main scanning by the laser beam and preventing a color misfit during formation of a color image is disclosed in Japanese Patent Laid-open No. 10-86446 (issued on Apr. 7, 1998), No. 10-86445 (issued on Apr. 7, 1998), No. 6-119571 (issued on May. 18, 1993) and No. 4-16969 (issued on Jan. 21, 1992).
However, there have been the following problems in the foregoing prior arts. Specifically, the photosensitive belt is made of a material with a mechanically low rigidity so that the photosensitive belt possesses an electrostatically charged property, so that such photosensitive belt is apt to be damaged. A non-linearity shape of the belt edge owing to the damage appears in an electrostatic latent image which is written to the photosensitive belt using the belt edge as a reference position.
Accordingly, there is a problem that when it is intended to record one straight line in a sub-scanning direction (driven direction of the photosensitive belt), an actually recorded image takes a jagged line shape similar to that of the belt edge.